Borehole seismic surveys permit geoscientists to analyze subsurface structural features, for example to evaluate the production potential beneath a plot of land. Generally, surface-induced seismic pulses from airgun sources generate seismic waves that travel underneath the earth's surface. The seismic waves are reflected off subsurface formations, and are usually recorded by downhole sensors. The recorded waves can be analyzed for geophysical information.
Surface-induced seismic pulses generated by airguns normally require a gun pit to be dug in the ground, which is then suitably lined and filled with water. The pits should be at least 10 m diameter and 8 m deep with 45 degree sloping sides (for a 2×150 cu. in. airgun source at 2000 psi). Problems can arise with this approach to generating seismic pulses because constructing an ideal gun pit is challenging. For example, any change in the shape of the pit may impact source signature consistency and shot quality, and ground conditions may lead to pit wall collapse or a buildup of loose soil/sand directly beneath the air gun during use. Liners often fail to protect against this damage by tilting, buckling, and/or cracking and potentially ultimately collapsing under continuous firing of the airgun.